This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of an aqueous solution of a surface-active ether-linked sulfonate.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding, which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus, it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant waterflooding or as low tension waterflooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. Thus far, many such waterflooding applications have employed anionic surfactants. One problem encountered in waterflooding with certain of the anionic surfactants such as petroleum sulfonates is the lack of stability of these surfactants in a so-called "high brine" environment. These surfactants tend to precipitate from solution in the presence of monovalent salts such as sodium chloride in concentrations in excess of about 2 to 3 weight percent and in the presence of much lower concentrations of divalent metal ions such as calcium and magnesium ions. In view of this disability of the petroleum sulfonates and similar anionic surfactants, various surfactant formulations which contain anionic ether-linked sulfates that tolerate high salinities and/or high divalent metal ion concentrations have been proposed for use in high brine environments. Thus, U.S. Pat. No. 3,508,612 to Reisberg et al. discloses a low tension waterflooding process employing a calcium-compatible anionic-anionic surfactant system containing an organic sulfonate such as a petroleum sulfonate and an ether-linked sulfate, i.e. a sulfated oxyalkylated alcohol or a sulfated polyoxyalkylated alkyl phenol.
Another waterflooding technique involving the use of calcium-compatible surfactant systems containing an ether-linked anionic surfactant is disclosed in U.S. Pat. No. 3,827,497 to Dycus et al. In this process, a three-component or two-component surfactant system may be employed. The three-component system comprises an organic sulfonate surfactant such as a petroleum sulfonate, a polyalkylene glycol alkyl ether, and an ether-linked surfactant which is a salt of a sulfonated or sulfated oxyalkylated alcohol. The two-component system comprises an organic sulfonate surfactant and an ether-linked surfactant which is a salt of a sulfonated oxyalkylated alcohol. These surfactant systems may be employed in a brine solution which, as noted in column 3, will usually contain about 0.5-8 percent sodium chloride and will often contain 50-50,000 ppm polyvalent metal ions such as calcium and/or magnesium ions. The sulfated or sulfonated oxyalkylated alcohols may be derived from aliphatic alcohols of 8-20 carbon atoms or from alkyl phenols containing 5-20 carbon atoms per alkyl group. The oxyalkyl moiety in the ether-linked surfactant usually is derived from ethylene oxide although other lower alkylene oxides containing 2-6 carbon atoms or mixtures thereof may be employed. Dycus et al. disclose that the sulfonated oxyalkylated alcohols in combination with the organic sulfonate have improved hydrolytic stability over similar formulations employing sulfated oxyalkylated alcohols as disclosed in the Reisberg et al. patent.
Another surfactant waterflooding process employing an ether-linked sulfonate surfactant in high salinity environments and in high temperature reservoirs, i.e. temperatures of 120.degree. F. or more, is disclosed in U.S. Pat. No. 3,977,471 to Gale et al. This patent discloses the use of an R.sub.1 hydrocarbyl ether-linked R.sub.2 hydrocarbyl sulfonate which is characterized as having a low rate of hydrolysis in the presence of aqueous liquids at higher reservoir temperatures. The R.sub.1 lipophilic base is provided by a benzene, toluene, or xylene radical having an alkyl substituent containing 6-24 carbon atoms and the R.sub.2 group linking the sulfonate group with the alkoxy ether group is a C.sub.1 -C.sub.8 alkyl, cycloalkyl, alkene or aryl radical. The R.sub.2 hydrocarbyl group may be substituted with a hydroxy group or a C.sub.1 -C.sub.8 aliphatic group. Dodecyl dimethyl benzene ether [EO].sub.4 propane sulfonate is specifically disclosed by Gale et al. The Gale et al. process is said to be particularly useful in reservoirs having high salinity brines, i.e. salinities of 2 percent or more, and the dodecyl dimethyl benzene ether [EO].sub.4 propane sulfonate is said to be stable in saline solutions containing from 7-14 weight percent sodium chloride and to not hydrolyze in water at 150.degree. F. for at least 6 months.
Gale et al. also disclose the use of the ether-linked sulfonates in liquid hydrocarbon solvents or in microemulsions. The microemulsion contains a refined or crude oil, an aqueous medium, and the ether-linked sulfonate. In addition, the microemulsion may contain a co-surfactant. Particularly effective co-surfactants are said to include alcohols, ethoxylated alcohols, sulfated ethoxylated alcohols, sulfonated ethoxylated alcohols, ethoxylated phenols, sulfated ethoxylated phenols and synthetic sulfonates. Alcohols disclosed in Gale et al. for use as microemulsion co-surfactants include C.sub.3 -C.sub.20 aliphatic alcohols such as isopropanol, isobutanol, tertiary butanol, amyl alcohols, hexanols, octanols, and dodecanols. The patentees specifically disclose flooding with microemulsion systems containing 91 percent to 93 percent water, containing 6 percent sodium chloride, 5 percent oil, 1 percent to 3 percent dodecyl dimethyl benzene ether [EO].sub.4 propane sulfonate, and 1 percent butyl alcohol.
Yet another surfactant waterflooding process involving the use of ether-linked sulfonates is disclosed in U.S. Pat. No. 4,018,278 to Shupe. The ether-linked surfactants employed in Shupe include sulfonated polyethoxylated aliphatic alcohols and sulfonated polyethoxylated alkyl phenols. The patentee discloses the use of the ether-linked sulfonates alone or as a co-surfactant with anionic surfactants such as petroleum sulfonates and in high brine environments, e.g. in a solution having a salinity of 183,000 ppm and a total hardness of 9400 ppm. This surfactant system is said to exhibit good thermal stability and to be particularly useful at temperatures in excess of 120.degree. F.
Additional disclosures involving the use of ether-linked sulfonates in surfactant waterflooding are found in U.S. Pat. No. 4,066,124 to Carlin et al. and U.S. Pat. No. 4,077,471 to Shupe et al. Carlin et al. disclose the use of alkyl or alkylaryl polyethoxyalkyl sulfonates as solubilizing co-surfactants in conjunction with predominantly water-soluble and partially oil-soluble petroleum sulfonates of dissimilar average equivalent weights. Shupe et al. disclose the use of a surfactant blend of a water-insoluble ethoxylated alkylaryl or ethoxylated aliphatic compound and a water-soluble alkyl or alkylaryl polyalkoxyalkyl sulfonate in which the ether linkage is provided by ethoxy groups or a mixture of ethoxy and propoxy groups with the ethoxy groups predominating. Both Carlin et al. and Shupe et al. describe their surfactant formulations as being stable in high brine and high temperature environments with Shupe et al. specifying the use of their process at formation temperatures ranging from about 70.degree. F. to about 300.degree. F.